The invention relates to arsenic-specific stains, kits for detecting arsenic, and processes for detecting arsenic. Arsenic-treated wood is identified thereby with minimal interference from phosphate. Processes for making arsenic-specific stains are also provided.
Preservative chemicals are added to wood products in order to prevent their biological deterioration from insects and fungi. In warm and humid climates, wood exposed to the outdoor environment will last only one to two years but its structural integrity can be maintained through 25 to 40 years by chemical treatment (Stalker, 1993; Cooper, 1993). Durability will depend upon the amount of chemical added to the wood, its use, and local climate conditions. A term used by the wood treatment industry to describe the amount of chemical added is “retention.” The lower the retention, the lower the amount of chemical added to the wood per unit volume of wood. Treatment at lower retention levels is used for wood intended for above ground and ground contact applications, whereas treatment at higher retention levels is used for wood intended as load-bearing supports for a structural member or for marine submersion (AWPA, 2003).
Wood preservatives can be separated into two broad categories: oilborne preservatives and waterborne preservatives (Milton, 1995). Wood treated with oil-borne preservatives are almost exclusively utilized for outdoor industrial applications since they are oily to the touch and in many cases have odors associated with them. The waterborne wood preservatives, also known as metal-based preservatives, are typically composed of metal oxides and, in some cases, an added organic co-biocide. Treated wood used in residential applications is almost exclusively treated with waterborne preservatives. Wood treated with waterborne preservatives is preferred for residential applications because, in contrast to their oilborne counterparts, the product can be painted, does not have an odor, and is dry to the touch.
Within the U.S., the metals typically used in wood preservative formulations include copper, chromium, arsenic, and boron (AWPA, 2003). The most common wood preservative through the 1980's and 1990's contained arsenic and included, predominantly, chromated copper arsenate (CCA) and, secondarily, acid copper zinc arsenate (ACZA) (AWPI, 1996; Micklewright, 1998). As of Jan. 2, 2004, the wood treatment industry voluntarily withdrew from manufacturing arsenic-treated wood for products intended for most residential uses (EPA, 2002). Exemptions to the ban include lumber and timber for salt water use only, piles, poles, plywood, wood for highway construction, poles/piles/posts used as structural members on farms and plywood used on farms, wood for marine construction, round poles/posts used in building construction, sawn timber used to support residential and commercial structures, sawn crossarms, structural glue laminated members and laminations before gluing, structural composite lumber, and shakes and shingles. Thus, the phase out will not result in the complete absence of arsenic-treated wood because the products listed above will likely continue to be treated with CCA or AZCA.
Phase out of arsenic-treated wood was initiated, in part, by growing public concerns about the possible adverse health effects associated with the arsenic contained in treated wood and with the advent of non-arsenic alternatives. Exposure to the chemicals contained in treated wood may occur directly through contact with the wood surface (Shibata et al., 2004; Stilwell, 2003; U.S. CPSC, 2003; ACC, 2003) or indirectly due to the leaching of the chemicals from the wood and subsequent environmental contamination (Stilwell & Gorny, 1997; Townsend et al., 2003a). The alternatives to CCA and ACZA typically contain higher concentrations of copper and an organic co-biocide. These alternatives do not contain arsenic as an active ingredient and so the concentration of arsenic within the treated wood product is low, typically near background concentrations for untreated wood (Table 1). Commercially available copper-based alternatives that can be used in residential outdoor settings include: alkaline copper quat (ACQ) and copper boron azole (CBA). In the past, other copper-based preservatives that have been sold in relatively small quantities in the U.S. include copper citrate (CC) and copper dimethydithiocarbamate (CDDC). Currently, the CC and CDDC alternatives are not marketed in the U.S. Borate-treated wood products are generally replacing CCA-treated wood in areas where treated wood is used indoors for added termite protection. Borate-treated wood is not recommended for outdoor use (AWPA, 2003) due to its tendency to leach out of the wood when wet. Some borate-treated products are generally well-fixed to the wood. The borate in these products is typically added in an insoluble powder form as zinc borate. Because the borate is added as a solid, treatment is limited to composite wood products such as oriented strand board, particleboard, and flakeboard. Since copper and boron are considered to be significantly less toxic to humans than arsenic, the alternatives are considered to be more environmentally acceptable and more favorable when contact with humans is likely. Concerns have been raised however due to the higher aquatic toxicity of copper relative to arsenic (Weis & Weis, 1995; Stook et al., 2004). Wood treated with copper-based alternatives is not recommended within sensitive aquatic environments (Stook et al., 2005).
TABLE 1Metal Concentrations of Common Metal-Based Preservatives inComparison to Untreated Wood.RetentionIndoorLevels forAbove Ground RetentionUseMarineLevelsInorganicSubmersionbMetalUntreatedCCAACZAACQCBABoronCCAPreservativeWood(4 kg/m3)(4 kg/m3)(4 kg/m3)(3.3 kg/m3)(4.5 kg/m3,e)(40 kg/m3)Arsenic,<81,8001,300NAcNANA18,000mg/kgaChromium,<222,000NANANANA20,000mg/kgCopper, mg/kg<41,2003,1004,3003,200NA12,000Zinc, mg/kg—NA1,600NANANANABoron, mg/kg—NANANAd  5702,800NAamg of metal per kilogram of wood. Concentrations were computed using the typical density of Southern Yellow Pine of 500 kg/m3 (AWPA, 2003).bACQ-, CBA- and borate-treated wood not standardized for marine submersion. ACZA is standardized for marine submersion.cNA = Not an active ingredient added to the wood.dBoron is added to the ACQ formulation as an anti-corrosion agent. It is not added as an active ingredient.eRetention level corresponds to formulation providing resistance to Formosan termites.
Although the manufacture of CCA-treated wood for residential applications has been greatly restricted as of 2004, there still exists a large inventory of CCA-treated wood currently in use due to the long service life of the treated wood product (Solo-Gabriele & Townsend, 1999). As a result of this inventory, there is a need to identify whether arsenic-treated wood is contained within existing structures so toxic exposure can be reduced. For example, recent risk assessments indicate that children who routinely play on CCA-treated playground equipment and decks may increase their risk of cancer by a margin greater than the acceptable 1 per million level due to the possible ingestion of arsenic from surface residues on treated wood (Roberts & Ochoa, 2001; CPSC, 2003; Maas et al., 2004; Dang et al., 2003). In such situations, in particular for structures frequented by children, it will be important to determine whether or not the wood contains arsenic so that measures can be taken to remediate existing wood structures. Remediation may include coating the structure with an oil-based penetrating stain (Stilwell, 1998; Feist & Ross, 1995; Maas et al., 2002; Lebow et al., 2003; Cooper et al., 1995; Cooper & Ung, 1997; EPA, 2004). Other concerns may arise when existing structures undergo cleaning and painting for aesthetic reasons. In such cases it would be helpful to know what chemicals are contained within the wood so that proper safety equipment is utilized and that care is taken in choosing the appropriate cleaners for the wood, since some oxidant-based cleaning products have a tendency to convert the chromium within CCA to a more toxic form (Taylor et al., 1998).
In addition to identifying CCA within structures that are currently being used, CCA-treated wood should be properly identified during its disposal. In Florida, construction and demolition (C&D) wood is in many cases recycled into consumer mulches (Tolaymat et al., 2000). C&D wood mulch must be essentially free from arsenic-treated wood in order for it to pass regulatory guidelines for recycling (Townsend et al., 2003b). But assuring that C&D wood mulch is free from CCA is not easy since it requires the rapid identification of chemical treatment during sorting. Also, care must be taken when disposing of CCA-treated wood within landfills. CCA-treated wood should be preferentially disposed within landfills that have bottom liners versus those that do not. In such cases, metals that leach from CCA-treated wood during its decomposition can be captured by the bottom liner and can be subsequently managed, versus the uncontrolled loss of the metals from unlined landfills. In many instances, the ultimate disposal of wood is through combustion (Solo-Gabriele & Townsend, 1999). Combustion may occur within mass burn solid waste facilities, or in facilities capable of recovering energy during the combustion processes. In either case, it will be important for wood-burning facilities to control their incoming waste/fuel stream in order to better manage air emissions and resulting ash quality. This is true for all treated wood but, in particular, wood treated with arsenic-containing preservatives since metals like arsenic are not decomposed. During combustion, the metals are usually concentrated in the ash or lost through air emissions in the event that air pollution control devices are not employed. Identification of preservatives contained within wood is thus important during disposal.
Another practical issue that requires the identification of treated wood products is the need for special fastener systems for wood treated with ACQ. ACQ-treated wood is generally more corrosive to metal fastener systems than CCA-treated wood (CSI, 1995), requiring either stainless steel or hot-dipped galvanized metal fasteners. As a result, during the construction process, it will be important to properly segregate ACQ-treated wood from CCA-treated wood so that the proper fastener systems can be utilized with each wood product. The need for such sorting was emphasized when a south Florida homebuilder's stockpiles of ACQ-treated wood were inadvertently, mixed with CCA-treated wood. There was a need to sort the stockpiles and reconfirm whether existing structures were constructed of CCA- or ACQ-treated wood to assure that the correct fasteners were used.
We focus on processes to identify whether or not wood is treated with arsenic-containing preservatives. Visualization techniques that can be utilized by simply looking at the shape and color of the wood to make an initial judgment concerning whether or not it is treated. Since simple visual techniques are not always accurate, augmentation technologies can be employed in the field for rapid identification of the metals contained in treated wood. Rapid identification techniques include the use of chemical stains, arsenic test kits, x-ray, and laser technologies. We describe a stain which is arsenate specific, minimizes interference by phosphate, and can quickly and inexpensively detect arsenic-treated wood in the field.
Visualization Techniques
When approaching wood which was used in outdoor structures or applicable indoor settings, the initial assumption is to consider it treated. When observing wood in the disposal stream, look for evidence that the disposed wood was part of a fence or a dock. This is readily observable when the structures have not been completely demolished upon arrival at the disposal facility. If the origin of the wood is not readily noted, look for end tags which may still be present on construction wood. These end tags will indicate the preservative used to treat the wood. The presence of barnacles on wood will indicate that it was used in marine applications and is likely treated. If the wood is incised, it is also treated. Incising is a process by which uniform cuts are made in the wood to improve the penetration of the preservative during treatment. Treated industrial wood products can usually be identified based upon their large dimensions. For example the typical dimensions of railroad ties are 0.2 m×0.2 m×2.6 m, and utility poles are typically 30 cm in diameter. Both industrial products are almost exclusively treated. In some cases, landscape timbers can be identified by their shape which in many cases is characterized by rounded edges for decorative purposes. Size and shape of wood is not a very good indicator of treatment for most residential wood which is primarily composed of lumbers, timbers, and plywood. Lumbers, timbers, and plywood can be found either treated or untreated, so identification of treated wood among wood products with these dimensions is more difficult.
Another simple visual method that can be used to preliminarily identify treated wood is to look at the color of the wood. Different wood species are characterized by different colors. The most common wood species used for treatment in the southern, northern, and midwestern U.S. is Southern Yellow Pine (SYP), which is naturally yellow in color. When SYP is treated with a preservative such as CCA, ACQ, or CBA, the color can vary from a light olive green to an intense green depending upon the amount of preservative added to the wood. The green color is due to the copper, which for wood treated at low retention levels can correspond to a very subtle color differential. Another distinct color change is observed for ACZA-treated Douglas Fir. In addition to incisions made during treatment, ACZA tends to react with specific compounds in Douglas Fir to produce a dark brown color, which is readily identifiable within the disposal sector (Ruddick & Xie, 1994).
Once the wood has been in-service and is weathered, the olive-green color from copper treatment is maintained only for wood treated at high retention levels (e.g., 40 kg/m3). The color of wood treated at low retention levels (e.g., 4 kg/m3) will change to a silver color upon weathering. This change in color occurs after only a year or two of weathering. Untreated wood generally weathers to the same silver color. As a result, identification of treated SYP wood based on color alone is difficult, in particular if the wood is weathered and was originally treated at a low retention level. Visual identification of copper-treated treated wood is further hampered if the wood comes from a demolition facility since dust and dirt will tend to mask the subtle green color of low retention level wood.
Because of limitations on visualization techniques for detecting treated wood, augmentation techniques are used to identify wood treated with copper or arsenic preservatives.
Augmentation Techniques
The simple visualization techniques described above are not accurate enough for identifying treated wood. Due to the phase-out of CCA-treated wood, more and more residential structures are being constructed of wood treated with non-arsenic copper-based alternatives (e.g., ACQ and CBA). Due to concerns associated with the toxicity of arsenic and the different fastener systems that are recommended for each of these wood types, there will be a growing need to improve methods by which CCA-treated wood can be distinguished from CBA- and ACQ-treated wood. CCA-, CBA-, and ACQ-treated wood are all olive-green in color and visual identification of each wood type (in the absence of end tags) is almost impossible. Limitations of visual sorting methods are further emphasized through studies by Blassino et al. (2002) who documented the quality of the “recyclable” wood piles at three C&D recycling facilities located in Florida. Two facilities practiced visual sorting and one did not. The two facilities that practiced visual sorting had between 9% and 10% CCA-treated wood, by weight, within their “untreated” wood pile, whereas the remaining facility had 30% CCA-treated wood in their wood pile. Although the number of C&D facilities evaluated by Blassino et al. (2002) was small, their study suggested that visual sorting does improve the quality of sorted wood and can potentially decrease the amount of CCA-treated wood entering the waste stream by about 60% to 70%. Furthermore, Blassino et al. (2002) visually sorted waste loads from a C&D facility based upon only the color and shape of the waste wood and then checked the accuracy of their sorts. The results of their study showed that visual sorting based upon the size and color of the wood correctly identified wood as “CCA treated” or “not CCA treated” 90% of the time, on average. Incorrect determinations occurred 10% of the time. Although visual sorting based on color and shape of the wood improves the quality of waste wood, such sorting is not accurate enough for assuring a wood waste essentially free of arsenic-treated wood.
There are several methods to augment the identification of wood treated with copper- and arsenic-containing preservatives (Homan & Militz, 1994). Many of them require laboratory analysis and are not suitable for field use. Typically, sawdust samples are provided to a laboratory which then processes the samples by dissolving the sawdust in an acidic solution and then the acidic solution is analyzed using sophisticated laboratory equipment (such as an atomic absorption spectrometer or inductively-coupled plasma atomic emission spectroscopy). If the sawdust sample is found to contain arsenic concentrations on the order of several hundred mg/kg or greater, then it has been treated with an arsenic-containing preservative. But laboratory analyses are expensive (typically on the order of $50 per sample) and require several days to weeks to obtain results.
Other more rapid methods for identifying treated wood can be separated into two categories: those characterized by low capital cost but are highly labor intensive and those that are semi-automated and characterized by high capital costs. Analysis times for labor intensive methods (e.g., PAN stain and generation/detection of arsine gas with a water test kit) vary from many seconds to many minutes. The analysis times for the semi-automated systems (e.g., laser and x-ray technologies) are typically on the order of a fraction of a second. These labor intensive methods use chemical stains or metal test kits, which readily identify the presence of a particular chemical. The cost of analysis by chemical stain or test kit is typically a fraction of a dollar per sample. The less labor-intensive semi-automated methods require the purchase of expensive equipment (many thousands of dollars). Such technologies are currently being evaluated for their potential to augment existing sorting techniques (based upon the green hue of treated wood) and to sort large quantities of wood using conveyor systems.
PAN stain is found to quickly react and creates a distinct color that could be readily identified in the field (Blassino et al., 2002). PAN is the abbreviation for the chemical 1-2(-pyridylazo)-2-napththol, an orange-red solid with a molecular formula of C15H11N3O (McMurry, 1992; Sandell & Onishi, 1978). It is used to determine the presence of almost all metals (40 to 50 in total), excluding alkali metals such as arsenic. During the reaction between a metal and PAN indicator, the metal bonds to the oxygen of the OH— group by replacing the hydrogen atom, and to pyridine and azo nitrogen atoms (Sandell & Onishi, 1978). The reaction with the non-alkali metals in CCA-treated wood produces color that is magenta to red; untreated wood turns orange in color.
A readily-available test kit (Product 17926, EM Science, Gibbstown, N.J.) is modified for the detection of arsenic in treated wood samples. The test kit utilized is sold commercially for analyzing arsenic in drinking water. Use of the kit is modified by adding a sawdust sample from the structure to be evaluated to a test tube. Water is then added to the test tube and chemicals that come with the kit (zinc and HCl) are added to the sawdust-and-water solution. The addition of these chemicals converts arsenic within the sawdust-and-water solution to arsine gas. This gas then reacts with a test strip (impregnated with mercury (II) bromide) to result in a color change from white to brown or black. In all cases, results from the arsenic test kit are consistent with the results from the laboratory analysis from the same structures. The test kit consistently provided positive readings for CCA-treated wood structures (ranging in concentration from 1400 to 3800 mg/kg As) and negative readings for structures made from untreated wood or non-arsenical copper-treated wood (less than 3 mg/kg As). The major drawbacks associated with this method are the need for laborious sample processing and the generation of toxic gas. As a result, this method may be used for evaluating only one or two structures in the field. This method is also not recommended for use by those inexperienced with the handling of chemicals, due to the use of toxic chemicals and the formation of arsine gas. It is likely that arsine gas generated from the analysis of treated wood samples is greater than the generation from drinking water samples due to the higher concentration of arsenic from the sawdust.
Laser induced breakdown spectroscopy (LIBS) is extremely rapid and can easily operate at 10 Hz (ten readings per second). A high-powered laser is directed towards the sample to create a spark (plasma flash) which vaporizes a small portion of the surface of the wood, provided that it is within a set distance (focal length) from the laser housing. The atoms within the plasma emit light characterized by different wavelengths; certain wavelengths of emitted light are unique to different elements in a phenomenon called atomic emission. Intensity of the emission is directly proportional to the amount of that element present in the original wood sample (Radziemski & Cremers, 1989). A pilot scale LIBS system was constructed and tested for sorting treated wood waste (Solo-Gabriele et al., 2004; Solo-Gabriele et al., 2001; Moskal & Hahn, 2002; Moskal, 2001). Once any coating is stripped away through successive laser pulses on the same area, LIBS could detect whether or not the wood sample was CCA treated. Excessive moisture in the wood interfered with the ability of the system to detect CCA-treated wood. Although the system is capable of detecting CCA on damp wood, it is not capable of detecting treatment on wood after it had been soaked in water.
X-ray fluorescence spectroscopy (XRF) has been used by the wood treatment industry as a means of checking the quality of the treated wood product (AWPA, 2003). Blassino et al. (2002) documented operating parameters from trials in the laboratory. These trials indicate that XRF can detect the presence of chromium, copper, and arsenic with arsenic providing the strongest signal among the three. The metals were detected even for the wood samples characterized by the lowest CCA retention levels (4 kg/m3). The same instrument was field tested at a C&D debris processing facility located in Sarasota County, FL (Solo-Gabriele et al., 2004).
But LIBS and XRF require expensive instrumentation and this limits the availability of detection to the locations where the instruments are located. Moreover, users of the instruments must be trained in their sophisticated operation. Such factors limit their widespread use for detecting arsenic-treated wood.
Therefore, it is an objective of the invention to provide a novel reagent for use as an arsenic-specific stain to detect arsenate extracted from wood suspected of being treated with preservative(s). The present invention is directed to an improved process for detecting arsenate in wood that addresses the aforementioned problems. It is quick, inexpensive, and non-sophisticated users are able to successfully segregate arsenic-treated wood (e.g., CCA or ACZA preservative). Other advantages and improvements are described below or would be apparent from the disclosure herein.